a) Field of the Invention
The present invention relates to a spindle motor which is used for data disk drive systems for driving data disks such as floppy disks, CD-ROMs. More specifically, it relates to a bonding structure for a hub and a motor shaft or a motor frame and a motor shaft in a spindle motor.
b) Description of the Related Art
Popular spindle motors, used in a data disk drive system include the "rotary shaft" type and "fixed shaft" type. In the "rotary shaft" type, a hub for securing a data disk is attached to a motor shaft which is rotatably supported by the motor frame. In the "fixed shaft" type, a hub is rotatably supported by a motor shaft which is fixed onto a motor frame. In each type, a data disk is driven in sequence, then a spindle motor is driven to rotate a hub on which the data disk is secured.
Now, the hub, or motor frame and motor frames are made from different members and have different rigidity, hardness, and accuracy. In general, they are manufactured separately and are mutually bonded by means of adhesive bonding, press-fitting or shrink-fitting.
On the other hand, a spindle motor in which a motor shaft is formed integral with a hub, is disclosed in Japanese patent Kokai H4-244756.
The bonding structure of conventional technology in which a motor shaft and a hub or a motor frame are made up of different members has the following problems. First, in order to ensure the bonding strength between these members, a large fitting area is required between these members. If a motor shaft of the "straight" type, whose outer diameter is identical to other members is used, the outer diameter is kept constant. In order to increase the fitting area, the fitting portion needs to be larger in the axial direction. As a result, a longer motor shaft will be required. The thickness of the motor shaft is limited by applications to a given dimension. If a longer motor shaft is used, a thinner bearing will be required for supporting the motor shaft. A thinner bearing unfavorably decreases the rigidity of the motor shaft.
A "step motor shaft" which has a fitting portion of a larger diameter may provide the larger fitting area without increasing its length. However, the "step motor shaft" is extremely expensive compared to the "straight" motor shaft. This significantly increases the price of a spindle motor.
On the other hand, when the motor shaft is formed integral with a hub, the following problems will be encountered. That is, some space is required near the border between the hub and motor shaft such that a cutting tool can machine thereon. It is difficult to machine a motor shaft which is formed integral with a hub. The motor shaft may not be machined with a desired accuracy. Also, integrating a motor shaft and hub takes a long time, which is inefficient in terms of productivity. As a result, a spindle motor whose motor shaft is formed integral with a hub become expensive.